A drug containing biologics, such as an influenza vaccine, is usually subcutaneously administered into a hypodermal tissue or intramuscularly administered into a muscular tissue deeper than a hypodermal tissue. Recently, it has been understood that a dosage of vaccine can be reduced by selecting an upper layer of skin, such as the epidermis and dermis, where a large number of immunocompetent cells exists as a target of vaccine administration. A skin includes an epidermis, which is a layer with a thickness of 50 to 200 μm from the surface of skin, a dermis, which is a layer with a thickness of 1.5 to 3.5 mm continuing from the epidermis, and a hypodermal tissue existing deeper than the dermis. The upper layer of skin is quite shallow to be targeted from the surface of a skin.
A known syringe for injecting a small amount of a drug into the upper layer of skin is configured with a tube body for containing the drug and an injection needle assembly including an injection needle, which are separately prepared. Immediately before injection, the injection needle assembly is attached to a drug discharge tube on the front end of the tube body.
For example, Japanese Patent Application Publication No. 2011-212185 (“Patent Literature 1”) discloses a drug injection device including a tube body having a liquid chamber for containing a drug, a pusher configured with a gasket attached to a plunger so as to move inside the tube body, a tubular gripping portion provided around the outer periphery of the tube body, a connector connecting the tube body and the end portion of the gripping portion, and an injection needle assembly including an injection needle with a needle hub that holds a needle tube with an elastic body, where a drug discharge tube on the front end of the tube body is pushed into a fitting hole of the injection needle assembly to be attached.
The injection needle assembly is attached to the fitting hole by a taper fitting. Specifically, the drug discharge tube of the tube body is formed to have a male taper shape, in which the outer diameter gradually decreases toward the distal end. Meanwhile, the injection needle assembly is provided with a sleeve hole, or a fitting hole, formed to have a female taper shape that has a size corresponding to the drug discharge tube and in which the inner diameter gradually decreases from the opening of the fitting hole. The male taper and the female taper are formed to have the same taper ratio. When the drug discharge tube is pushed hard into the fitting hole, the taper surfaces are pressed against each other to be secured with a liquid-tight fit. In this state, the drug discharge tube on the front end of the tube body is exactly in contact with the elastic body inside a fitting sleeve of the tube body of the needle hub to prevent leakage.